1. Field of the Invention
The present invention relates to a packet communication system that uses beam forming and enables transmission and reception of multiple packets exclusively in time and space, and relates to a communication method for the packet communication system, and a program readable by the packet communication system for executing the method.
2. Description of Related Art
In recent years, use of beam forming has gradually increased in radio communication systems using radio waves. Beam forming is a technique for reducing radio wave interference between base stations and terminals so that the radio wave can be transmitted for a longer distance (for example, see International Publication No. WO/02/052748). Use of this technique enables propagation of electromagnetic wave energy with a certain directivity, so that transmission distance, space utilization efficiency, power efficiency and the like can be enhanced.
Beam forming is achieved by combining two techniques: null steering and beam steering. Null steering is a technique for preventing a base station from transmitting and receiving a radio wave in and from a direction where another base station interfering with the first base station exists. Beam steering is a technique for intensely transmitting a radio wave of a base station in a specific direction.
In null steering, an antenna including multiple antenna elements arranged in a certain order is used to give a certain directivity to a radio wave, so that radio waves can be transmitted and received without interfering with those transmitted from other base stations or terminals. Meanwhile, in beam steering, a radio wave can be transmitted intensely by varying the phase and power of each of the signals transmitted from multiple antenna elements.
However, in a system where communications are made by use of beam forming, a problem exists of how to perform efficient synchronization and broadcasting among individual transceivers.
Since data is exchanged by synchronizing a data signal and a local clock signal, transceivers which are to exchange the data need to operate at the same clock speed. In a case of transmitting a packet to a destination address through multiple transceivers, all of the transceivers need to operate at the same clock speed. To make all of the transceivers operate at the same clock, synchronization signals are transmitted to the transceivers, so that each of the transceivers may use the synchronization signal to synchronize with the local clock signal for reading data. Generally, synchronization signals can be transmitted efficiently by broadcasting. However, in communication by use of beam forming, synchronization signals cannot be transmitted efficiently because radio waves are given a certain directivity and are transmitted only to specific transceivers.
Moreover, each transceiver transmits a broadcast signal to an unspecified number of transceivers, acquires address information, routing information, channel information and the like of communicable transceivers by receiving responses from the transceivers, and stores the acquired information. The address information includes a MAC address or an IP address, the routing information includes a routing table recording optimal routes to various destinations, and the channel information includes a frequency band. Each transceiver is capable of periodically transmitting the broadcast signal to acquire information that the transceiver does not store therein, and use the new information to update information stored therein. However, in communication by use of beam forming, broadcast signals cannot be transmitted efficiently as similar to the above case.
Against this background, there is a method in which a master controller is used to schedule all communications with all devices, and distribute synchronization signals and broadcast signals to individual devices (for example, see Japanese Patent Application Publication No. 2008-061256). However, with this method, if the number of devices increases, overhead is increased proportionally to the number of devices. Moreover, every device needs to communicate with the master controller, and thus arrangement of the devices is restricted.
There is also a method for stopping the beam forming during synchronization and broadcasting. In this method, the synchronization and broadcasting can be performed efficiently since radio waves do not have directivity. However, the transmission distance of a signal is shortened in this method, or the data rate needs to be lowered in order to keep the same transmission distance, as in the case of using the beam forming technique.